The invention relates to a continuously operating centrifugal or centrifuge for centrifuging sugar massecuites and for remashing the centrifuged sugar. Such centrifuges may include conventionally a ring conduit with nozzles for directing mixing liquid at the upper wide end of the centrifugal basket.
The ring conduit is arranged in a housing having a cover above a frustum-shaped centrifugal basket equipped with screens and rotating about a vertical axis of rotation. A conical baffle member is arranged radially outside the upper edge of the centrifugal basket for intercepting or catching the sugar crystals.
The idea of remashing the sugar centrifuged in a continuously operating sugar centrifuge before it leaves the centrifugal in order to thus obtain a new massecuite which may readily be fed without any further treatment, especially without any further mixing treatment into a following centrifugal, has been mentioned in several publications. These publications, however, preferably emphasize other problems. For example: the prevention of damage to the sugar crystal, or the prevention of the formation of lumps or of so-called tailings, or the discharge of the centrifuged sugar in a liquid medium have been emphasized heretofore. Thus, it is not surprising, that such known, prior art centrifugals are not capable of performing a technologically satisfactory mashing work.
The U.S. Pat. No. 2,883,054, for example, is concerned with a continuously operating horizontal centrifugal, having a basket, the rotational axis of which extends horizontally. The wide end of the frustum-shaped centrifugal basket merges into a sugar collecting compartment, which is designed as a torus opening towards the axis of rotation of the centrifugal basket. A ring conduit is arranged inside the sugar collecting compartment and concentric to the outer circular arc of said torus, said ring conduit is connected to a supply line for a treating agent and has exit orifices for said treating agent. The orifices are directed at the inner surface of the torus. The treating agent to be used may be either solvent liquid for again solving, or a liquor for remashing, or hot compressed air for drying the centrifuged sugar.
The remashing may at best be performed in this known centrifugal to the extent that a product of an inhomogeneous consistency is obtained, because the crystal mash formed in the top region of the sugar collecting compartment flows downwardly along both sides due to gravity, whereby the mash is hit on one side by the sugar crystals moving in the discharge direction, while on the other side the mash is hit by sugar crystals flying in the opposite direction which decelerate the mash and thus enriches it with sugar to a substantially greater extent than on the first side. Thus, at the lowest point of the sugar collecting compartment, two streams of different consistencies meet, which do not mix into a homogeneous product without any subsequent mechanical treatment. Furthermore, small or larger quantities of material cannot be prevented from coming off the top of the sugar collecting compartment and falling down. These material quantities might differ considerably in their consistencies from the confluent material streams, whereby the inhomogeneity of the product is also substantially increased.
The primary problem solved by the sugar centrifugal according to German Patent Publication (DAS) No. 2,025,828 is to completely prevent the formation of lumps in centrifuging the crystals in continuously operating centrifugals. To solve this problem, the upper end of the centrifugal basket is encircled by a fixed intercepting ring, wherein the introduced liquid rotates with a circumferential speed which is lower than the speed of the centrifugal basket. The rotation of the liquid ring or of the liquid-crystal mixture formed in this ring is produced by the air friction caused by a discharge flange which, depending to the type product, dips more or less into the intercepting ring and is secured to the centrifugal basket. The liquid may be introduced into the intercepting ring in different ways. The liquid-crystal mixture is removed from the ring at a single point on the circumference of the intercepting ring through an outlet connection.
This known centrifugal of German Patent Publication No. 2,025,828 leaves room for improvement in its structural features. When switching from one to another production, a great deal of assembly work is necessary for fitting the required discharge flange. For this purpose the intercepting ring must be disassembled among other required work. Routine cleaning work is similarly troublesome. Without subsequent mechanical mixing it is not possible to feed the produced liquid-crystal mixture as a massecuite directly into a following centrifugal because a high rotational speed of the intercepting ring results in sedimentation in said ring, i.e., de-mixing in the intercepting ring, whereas lower speeds do not produce a mixing energy high enough to ensure the homogeneity of the liquid-crystal mixture at the outlet connection.
The U.S. Pat. No. 3,238,063 describes a centrifugal wherein a circular pipe is arranged at some distance from the upper wide end of a frustum-shaped centrifugal basket, that is, above a flat end ring attached to the upper basket end. Mixing liquid or--if the sugar shall be remashed--mashing liquid is sprayed out of the circular pipe at the sugar crystals flying over the end ring. A vertically suspended baffle plate made of elastic material is arranged radially outside of the end ring. The baffle plate is intended to intercept the sugar crystals without destroying the crystals and without the formation of bumps. However, a product that may be fed directly as a massecuite into a following centrifugal without any further, treatment, especially mixing, cannot be produced, because there are too many ways in which, quite accidentally, differently concentrated liquid-crystal mixtures may result or occur. For example, the mashing liquid is sprayed onto the sugar crystals from a comparatively large distance, namely, from the upper edge of the centrifugal basket in the radial direction of the basket whereby a widely fanned out spray pattern is obtained. Such a spray pattern permits the very strong air turbulences prevailing in continuously operating centrifugals to attack the sprayed-on liquid over large areas and to prevent, or unpredictably impair the desired intimate mixing of crystals and liquid. The vertically suspended baffle plate also produces inhomogeneity. As a result of the air turbulences which more or less break up the mixing liquid, the sugar crystals hit the baffle plate in differently moistened states. This fact aside, varying adhesivenesses also result in different situations despite the elasticity of the baffle plate. Part of the sugar flows off downwardly together with the mashing liquid, part of the sugar temporarily adheres and steadily picks up other centrifuged sugar crystals whereby the sugar is enriched with crystals, while the impinging liquid separates out and flows off downwardly. The result is an inhomogeneous product. Past experiences have shown that elastic materials for the baffle plates can withstand the wearing stress brought about by the sugar crystals impinging on the plate at a high velocity for a very short time only, which made practical use impossible. However, if the elastic baffle plate in this centrifugal is replaced by a steel plate, the crystals are destroyed and the product is not suitable as a massecuite.
U.S. Pat. No. 4,008,098 discloses a continuously operating sugar centrifuge wherein the sugar crystals passing over the upper edge of the centrifugal basket are sprayed from above with solving liquid and are then intercepted by a fixed wall or baffle ring which is arranged in an inclined position relative to the direction of crystal flight. The inclination of this baffle ring is such, however, that the material is deflected downwardly from the point of interception in such a way that it is impossible for the material to return to the interception area. The hard surface of the baffle ring contributes to crystal crushing and its inclination ensures that the crystals always hit the hard crystal-crushing surface. This known construction of a centrifugal may be suitable for returning the sugar into solution but it is not suitable for a most gentle crystal handling as is required for mashing. The baffle ring in the prior art centrifuge of U.S. Pat. No. 4,008,098 causes crystal destruction. Even the so-called "pad" formation which is known as such does not lent itself to preventing crystal destructions because in known centrifuges wherein the centrifuged sugar was to be mashed again the pad formation has been the cause of an uncontrollable inhomogeneous consistency of the produced mash.